1. Field of the Invention
The present invention relates to a high strength hot-dip galvanized steel sheet having tensile strength above 700 MPa, and particularly to a high strength hot-dip galvanized steel sheet that hardly induces softening at heat-affected zone (HAZ) during welding and that has excellent formability, and a method for manufacturing thereof.
2. Description of Related Arts
High strength hot-dip galvanized steel sheets having higher than 440 MPa of tensile strength are used in wide fields including construction materials, machine and structural members, and structural members of automobiles owing to the excellent corrosion resistance and the high strength.
Responding to ever-increasing severity of requirements on formability in recent years, various technologies to improve the formability of that type of high strength hot-dip galvanized steel sheet have been introduced. For example, according to JP-A-5-311244, (the term “JP-A” referred herein signifies the “unexamined Japanese patent publication”), a Si—Mn—P bearing hot-rolled steel sheet is heated to temperatures at or above Ac1 transformation point in a continuous hot-dip galvanizing line, and the heated steel sheet is quenched to Ms point or below to generate martensite over the whole or in a part thereof, then the martensite is tempered using the heat of the hot-dip galvanizing bath and of the alloying furnace. According to JP-A-7-54051, a hot-rolled steel sheet of Mn—P—Nb(—Ti) bearing is coiled at a low temperature after hot-rolled, which steel sheet is then subjected to hot-dip galvanizing to let pearlites or cementites disperse finely in the fine ferrite matrix to improve the stretch flangeability.
On the other hand, structural members of automobiles have recently been adopting steel sheets of different strength or different thickness which are joined together by laser welding or mush-seam welding, called “Tailor Welded Blanks” (TWB). Thus, the characteristics of welded part are also emphasized.
The high strength hot-dip galvanized steel sheet manufactured by the method disclosed in JP-A-5-311244 aiming at the improvement of formability of the steel sheet itself, however, is not applicable to the structural members of automobiles or the like because the softening at HAZ likely occurs during welding to induce degradation of formability and strength at the welded part. It is because, though the mechanism of strengthening is based on the second phase obtained by rapid-cooling austenite, the ferrite and the second phase are not fully homogeneously refined. The term “second phase” referred herein signifies a phase consisting of at least one structure selected from the group consisting of martensite and bainite. The high strength hot-dip galvanized steel sheet manufactured by the method disclosed in JP-A-7-54051 is difficult to stably have tensile strength exceeding 700 MPa, particularly above 780 MPa, because the structure thereof is a ferrite matrix with finely dispersed pearlites or cementites.